Genetic diversity,population structure and gene flow in native populations of a proposed biocontrol agent (Cornops aquaticum)

The semiaquatic grasshopper Cornops aquaticum is native to South America, with a distribution as far south as the Argentinean pampas and as far north as the Gulf of Mexico. This grasshopper is being proposed as a form of biological control agent for the invasive aquatic macrophyte (Eichhornia crassipes) in South Africa. The results of a molecular study (six microsatellite loci) conducted on 11 C. aquaticum populations is presented. Unlike in contiguous mainland South American populations, we found genetic diversity to be lowest in a South African quarantine population, with reduced values in two other isolated populations from South America. In addition, F_st/R_st/analysis of molecular variance and Bayesian cluster analysis suggested high levels of connectivity between mainland populations. The implications of these findings and their relationship to those of a recent morphological study suggest that the suitability of C. aquaticum as a form of biocontrol might be unpredictable at best. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 666–676.     

Polymerase chain reaction primers for microsatellite loci in the semi‐aquatic grasshopper,Cornops aquaticum

The semi‐aquatic grasshopper Cornops aquaticum is native to South America, with a distribution as far south as the Argentinian pampas and as far north as the Gulf of Mexico. This study reports the characterization of nine microsatellite loci for this species. To our knowledge, no molecular studies have been conducted on C. aquaticum, therefore, little is known about this species’ genetic diversity and how this relates to host specificity. These primers will allow further studies to elucidate the population history of C. aquaticum across both its native and introduced ranges.     

Assessing density–damage relationships between water hyacinth and its grasshopper herbivore

Plants are variable in their responses to insect herbivory. Experimental increases in densities of phytophagous insects can reveal the type of plant response to herbivory in terms of impact and compensatory ability. The relationship between insect density and plant damage of a grasshopper, Cornops aquaticum Brüner (Orthoptera: Acrididae: Tetrataeniini), a candidate biological control agent, and an invasive aquatic plant, water hyacinth, Eichhornia crassipes Mart. Solms‐Laubach (Pontederiaceae), was investigated to assess potential damage to the weed. The impact of different densities of male and female grasshoppers on E. crassipes growth parameters was determined in a quarantine glasshouse experiment. Damage curves indicated that the relationship between plant biomass reduction and insect density was curvilinear whereas leaf production was linear. Female C. aquaticum were more damaging than males, causing high rates of plant mortality before the end of the trial at densities of three and four per plant. Feeding by C. aquaticum significantly reduced the total plant biomass and the number of leaves produced, and female grasshoppers caused a greater reduction in the number of leaves produced by water hyacinth plants than males. Grasshopper herbivory suppressed vegetative reproduction in E. crassipes, suggesting C. aquaticum could contribute to a reduction in the density and spread of E. crassipes infestations. The results showed that E. crassipes vigour and productivity decreases with an increase in feeding intensity by the grasshopper. Cornops aquaticum should therefore be considered for release in South Africa based on its host specificity and potential impact on E. crassipes.     

VARIATION IN PREFERENCE AND SPECIFICITY IN MONOPHAGOUS AND OLIGOPHAGOUS SWALLOWTAIL BUTTERFLIES

Although variation in oviposition preference and specificity for host plants has been demonstrated within populations of a variety of oligophagous insect species, it is unknown whether genetic variation in host choice is lost within populations of monophagous species. Analysis of a locally monophagous butterfly species, Papilio oregonius, and a locally oligophagous species, P. zelicaon, showed significant variation in oviposition preference within populations of both species. Females of both species chose primarily their native hosts. Nonetheless, the percentages of eggs laid by individual females among the plant species and the number of plant species on which individual females laid eggs differed significantly among isofemale strains within populations. Moreover, some females within all isofemale strains of both species laid a few eggs on Foeniculum vulgare, an umbelliferous species that does not occur in the native habitats of these populations but is a host for Papilio species in other geographic areas. The results suggest that local monophagy and oligophagy in these species reflect the relative ranking among potential plant species. Both populations harbor variation in oviposition choice that could allow for host shifts if these populations invaded new habitats.     

Microsatellite DNA analysis of population structure in <Emphasis Type="Italic">Cornops aquaticum</Emphasis> (Orthoptera: Acrididae), over a cline for three Robertsonian translocations

The grasshopper Cornops aquaticum occurs between Mexico (23°N) and Uruguay and Central Argentina (35°S). It was recently introduced as a pest control agent of the neotropical water-hyacinth Eichhornia crassipes in South Africa. The information about the amount and distribution of genetic variability of the native populations may optimise the results of biological control programmes. Here we analyse microsatellite variability at the south of C. aquaticum’s distribution, coinciding with a cline for three polymorphic Robertsonian translocations along the Paraná River in order to: (1) estimate the amount of intrapopulation variation and its correlation with geographic/climatic variables, (2) infer interpopulation genetic variation and assess connectivity between local populations and (3) compare chromosome, morphometric and molecular variation patterns to analyse the probable causes involved in the maintenance of intraspecific variation. Our sample of 170 individuals of C. aquaticum from seven Argentine populations between latitudes 27°S to 34°S showed 211 alleles across seven microsatellite loci. Genetic diversity was estimated through average number of alleles, allelic richness, expected heterozygosity and observed heterozygosity. The analysis of molecular variance showed significant genetic differentiation among populations. Pairwise comparisons of F_ST/R_ST and Bayesian population assignment method and the discriminant analysis of principal components revealed that the two southernmost populations are more differentiated. Genetic diversity is negatively correlated with Southern latitude and with Robertsonian translocation frequencies. Our results showed that the Paraná River’s middle course populations are genetically undifferentiated and more genetically diverse than the highly chromosomally polymorphic downstream ones. The chromosomal polymorphisms are associated with increased body size in the direction in which larger size is adaptive. This may be relevant for C. aquaticum’s role as a pest control agent, since chromosome variability would enhance the ability of the species for a successful settlement in its new habitats, especially in temperate regions of the world.     

Phenological synchrony affects interaction strength of an exotic weevil with Platte thistle,a native host plant

Phenological synchrony of a consumer population with its resource populations is expected to affect interaction intensity. We quantified phenological variation and synchrony of populations of an invasive Eurasian flower head weevil, Rhinocyllus conicus, that consumes florets, ovules, and seeds of developing flower heads of a native North American thistle, Cirsium canescens, in Sand Hills prairie in Nebraska, USA. Variation in timing of adult activity among weevil populations was larger than variation in timing of flower head development among C. canescens populations, and it drove the observed variation in the phenological synchrony between weevil and host plant populations. Furthermore, the degree of phenological synchrony between populations was significant in explaining variation in weevil egg load on the newly acquired host plant. Because population growth of C. canescens is limited by predispersal seed losses to floral herbivores, variation in the synchrony of herbivore and plant flowering will affect the density of the plant population. These results provide strong quantitative support for the hypothesis that the synchrony of insect activity with plant resources can determine the magnitude of impact of floral herbivores on their host plant populations.     

Tetranychus evansi spider mite populations suppress tomato defenses to varying degrees

Plant defense suppression is an offensive strategy of herbivores, in which they manipulate plant physiological processes to increase their performance. Paradoxically, defense suppression does not always benefit the defense‐suppressing herbivores, because lowered plant defenses can also enhance the performance of competing herbivores and can expose herbivores to increased predation. Suppression of plant defense may therefore entail considerable ecological costs depending on the presence of competitors and natural enemies in a community. Hence, we hypothesize that the optimal magnitude of suppression differs among locations. To investigate this, we studied defense suppression across populations of Tetranychus evansi spider mites, a herbivore from South America that is an invasive pest of solanaceous plants including cultivated tomato, Solanum lycopersicum, in other parts of the world. We measured the level of expression of defense marker genes in tomato plants after infestation with mites from eleven different T. evansi populations. These populations were chosen across a range of native (South American) and non‐native (other continents) environments and from different host plant species. We found significant variation at three out of four defense marker genes, demonstrating that T. evansi populations suppress jasmonic acid‐ and salicylic acid‐dependent plant signaling pathways to varying degrees. While we found no indication that this variation in defense suppression was explained by differences in host plant species, invasive populations tended to suppress plant defense to a smaller extent than native populations. This may reflect either the genetic lineage of T. evansi—as all invasive populations we studied belong to one linage and both native populations to another—or the absence of specialized natural enemies in invasive T. evansi populations.     

Host plant use of Phyllotreta nemorum: do coadapted gene complexes play a role?

The view of (insect) populations as assemblages of local subpopulations connected by gene flow is gaining ground. In such structured populations, local adaptation may occur. In phytophagous insects, one way in which local adaptation has been demonstrated is by performing reciprocal transplant experiments where performance of insects on native and novel host plants are compared. Trade-offs are assumed to be responsible for a negative correlation in performance on alternative host plants. Due to mixed results of these experiments, the importance of trade-offs in host plant use of phytophagous insects has been under discussion. Here we propose that another genetic mechanism, the evolution of coadapted gene complexes, might also be associated with local adaptation. In this case, however, transplant experiments might not reveal any local adaptation until hybridization takes place. We review the results we have obtained in our work on the host plant use of the flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae), and propose a hypothesis involving coadapted genes to explain the distribution of genes that render P. nemorum resistant to defences of one of its host plants, Barbarea vulgaris R. Br. (Cruciferae).     

Enemy release does not increase performance of <Emphasis Type="Italic">Cirsium</Emphasis> <Emphasis Type="Italic">arvense</Emphasis> in New Zealand

Cirsium arvense (L.) Scop. (Californian, Canada, or creeping thistle) is an exotic perennial herb indigenous to Eurasia that successfully established in New Zealand (NZ) approximately 130 years ago. Presently, C. arvense is considered one of the worst invasive weeds in NZ arable and pastoral productions systems. A mechanism commonly invoked to explain the apparent increased vigour of introduced weeds is release from natural enemies. The enemy-release hypothesis (ERH) predicts that plants in an introduced range should experience reduced herbivory, particularly from specialists, and that release from this natural enemy pressure facilitates increased plant performance in the introduced range. In 2007, surveys were carried out in 13 populations in NZ (7 in the North Island and 6 in the South Island) and in 12 populations in central Europe to quantify and compare growth characteristics of C. arvense in its native versus introduced range. Altitude and mean annual precipitation for each population were used as covariates in an attempt to explain differences or similarities in plant traits among ranges. All plant traits varied significantly among populations within a range. Shoot dry weight was greater in the South Island compared to Europe, which is in line with the prediction of increased plant performance in the introduced range; however, this was explained by environmental conditions. Contrary to expectations, the North Island was not different from Europe for all plant traits measured, and after adjustment for covariates showed decreased shoot density and dry weight compared to the native range. Therefore, environmental factors appear to be more favourable for growth of C. arvense in both the North and South Islands. In accordance with the ERH, there was significantly greater endophagous herbivory in the capitula and stems of shoots in Europe compared to both NZ ranges. In NZ, capitulum attack from Rhinocyllus conicus was found only in the North Island, and no stem-mining attack was found anywhere in NZ. Thus, although C. arvense experiences significantly reduced natural enemy pressure in both the North and South Islands of NZ there is no evidence that it benefits from this enemy release.     

Variation in hostplant preference and performance by the milfoil weevil, Euhrychiopsis lecontei Dietz, exposed to native and exotic watermilfoils

Exotic plants often displace native plants and thus alter the availability of native hostplants for specialist herbivorous insects. The submersed aquatic weevil Euhrychiopsis lecontei Dietz is endemic to North America, but there are now source populations on the exotic Eurasian watermilfoil (Myriophyllum spicatum L.) as well as on the weevil's ancestral host, northern watermilfoil (Myriophyllum sibiricum Komarov). This provides an opportunity to examine a host range expansion in progress. To further define the host range of the weevil and to determine how population source and rearing plant influence host plant preference and performance, we conducted a series of preference and rearing experiments with weevils from two source populations reared on northern milfoil, on Eurasian milfoil, switched late in larval development from northern to Eurasian milfoil, and vice versa. We also included two rearing treatments with milfoils on which the weevil has not been documented: the native M. verticillatum L. and the exotic M. aquaticum Verd. Preference by weevils in the switched rearing treatments was similar to preferences exhibited by weevils reared solely on the second (later) milfoil species and an increase in preference for Eurasian milfoil was induced by adult exposure to Eurasian milfoil for 2 weeks. In contrast, sizes and development times of weevils in the switched rearing treatments were similar to sizes and development times exhibited by weevils reared solely on the first (early) milfoil species. These results indicate that preference by the milfoil weevil is determined late in larval development or later and Hopkins' host selection principle is not supported. However, size and development time were most affected by hostplant quality during larval development when larvae must acquire the resources needed for pupation. Oviposition preference in the milfoil weevil was a population attribute, not a fixed individual attribute and there was no significant variation in preference among individuals reared on northern milfoil, but significant variation in preference was detected among weevils reared on Eurasian milfoil. Weevils oviposited on all four milfoil species and completed development on three of them, but did not develop beyond the larval stage on M. aquaticum. Weevils reared on Eurasian milfoil developed faster and reached larger adult sizes than weevils in any other rearing treatment. The smallest sizes and longest development times were for weevils reared on the natives, northern milfoil and M. verticillatum. The milfoil weevil oviposits on an array of milfoil species and is unable to distinguish an unsuitable host (M. aquaticum) within this genus. The influence of rearing plant and adult exposure to Eurasian milfoil on hostplant preference suggests that host range expansion to novel congeners may occur more rapidly than predicted by models which assume that genetic variation is required. Significant variation among individuals in hostplant preference suggests the potential for a host shift to a plant for which E. lecontei appears pre-adapted.     

Can alien plants support generalist insect herbivores?

Simple rearing experiments were conducted to address two questions relevant to understanding how generalist lepidopteran herbivores interact with alien plants. Yellow-striped armyworm (Spodoptera ornithogalli), luna moth (Actias luna), bagworm (Thyridopteryx ephemeraeformis) and white-marked tussock moth (Orgyia leucostigma) were reared from egg to 5th instar on excised foliage in the laboratory to determine the degree to which highly polyphagous lepidopteran herbivores are physiologically capable of surviving and developing on the suite of alien plants naturalized in the mid-Atlantic. Actias luna larvae from a single population were similarly reared on a representative of each of the 25 native plant genera recorded as hosts for this species to compare the diet breadth of a local population with that listed over the entire geographic range of the species. With few exceptions, all four generalists either quickly starved or grew at an unsustainably low rate on alien foliage. Actias luna larvae survived for 18 days on only 44% of the native plants recorded as hosts over the entire range of this insect and thrived on only 7%. The data suggest that (1) alien plants are unlikely to produce as much generalist insect biomass as the native plants they replace and (2) Lepidoptera that qualify as generalists when host breadth is considered over their entire geographic range may express a far more specialized diet within local populations. Both of these conclusions support the hypothesis that alien plant invasions may seriously disrupt terrestrial food webs by reducing the insect biomass required by insectivores in higher trophic levels.     

Local adaptation and ecological genetics of host-plant specialization in a leaf beetle

The tendency of insect species to evolve specialization to one or a few plant species is probably a major reason for the remarkable diversity of herbivorous insects. The suggested explanations for this general trend toward specialization include a range of evolutionary mechanisms, whose relative importance is debated. Here we address two potentially important mechanisms: (i) how variation in the geographic distribution of host use may lead to the evolution of local adaptation and specialization; (ii) how selection for specialization may lead to the evolution of trade‐offs in performance between different hosts. We performed a quantitative genetic experiment of larval performance in three different populations of the alpine leaf beetle Oreina elongata reared on two of its main host plants. Due to differences in host availability, each population represents a distinctly different selective regime in terms of host use including selection for specialization on one or the other host as well as selection for utilizing both hosts during the larval stage. The results suggest that selection for specialization has lead to some degree of local adaptations in host use: both single‐host population had higher larval growth rate on their respective native host plant genus, while there was no difference between plant treatments in the two‐host population. However, differences between host plant treatments within populations were generally small and the degree of local adaptation in performance traits seems to be relatively limited. Genetic correlations in performance traits between the hosts ranged from zero in the two‐host population to significantly positive in the single‐host populations. This suggests that selection for specialization in single host populations typically also increased performance on the alternative host that is not naturally encountered. Moreover, the lack of a positive genetic correlation in the two host‐population give support for the hypothesis that performance trade‐offs between two host plants may typically evolve when a population have adapted to both these plants. We conclude that although there is selection for specialization in larval performance traits it seems as if the genetic architecture of these traits have limited the divergence between populations in relative performance on the two hosts.     

Impacts of Myriophyllum aquaticum invasion in a Mediterranean wetland on plant and macro-arthropod communities

The invasion by alien macrophytes in aquatic ecosystems may produce a strong alteration of the native aquatic vegetation leading to heavy impacts for both plant and faunal native diversity. Myriophyllum aquaticum is an aquatic plant native of Southern America, invasive in several part of the world. We studied the effects of M. aquaticum invasion on plant and macro-arthropod communities in the canals around a protected wetland in the Mediterranean basin. We sampled plant and macro-arthropod communities in 10 transects in invaded and non-invaded tracts of the canals. We assessed the differences in plant and macro-arthropod species richness, diversity, taxonomic diversity and species composition between invaded and non-invaded habitats by means of univariate and multivariate analyses. Our study shows a significant loss of plant diversity between non-invaded to invaded sites, leading to communities numerically and taxonomically impoverished and highly divergent in the species composition. We also detected significant differences in arthropod species composition between invaded and non-invaded transects. Some taxa such as mosquitoes and malacostraca were more frequent in the M. aquaticum-dominated stands. Furthermore, the study shows a positive relation between invaded habitats and juvenile individuals of the invasive alien crayfish Procambarus clarkii.     

Adaptation to an invasive host is driving the loss of a native ecotype

Locally adapted populations are often used as model systems for the early stages of ecological speciation, but most of these young divergent populations will never become complete species. The maintenance of local adaptation relies on the strength of natural selection overwhelming the homogenizing effects of gene flow; however, this balance may be readily upset in changing environments. Here I show that soapberry bugs (Jadera haematoloma) have lost adaptations to their native host plant (Cardiospermum corindum) and are regionally specializing on an invasive host (Koelreuteria elegans), collapsing a classic and well‐documented example of local adaptation. All populations that were adapted to the native host—including those still found on that host today—are now better adapted to the invasive host in multiple phenotypes. Weak differentiation remains in two traits, suggesting that homogenization across the region is incomplete. This study highlights the potential for adaptation to invasive species to disrupt native communities by swamping adaptation to native conditions through maladaptive gene flow.     

Release from native root herbivores and biotic resistance by soil pathogens in a new habitat both affect the alien <Emphasis Type="Italic">Ammophila arenaria</Emphasis> in South Africa

Many native communities contain exotic plants that pose a major threat to indigenous vegetation and ecosystem functioning. Therefore the enemy release hypothesis (ERH) and biotic resistance hypothesis (BRH) were examined in relation to the invasiveness of the introduced dune grass Ammophila arenaria in South Africa. To compare plant–soil feedback from the native habitat in Europe and the new habitat in South Africa, plants were grown in their own soil from both Europe and South Africa, as well as in sterilised and non-sterilised soils from a number of indigenous South African foredune plant species. While the soil feedback of most plant species supports the ERH, the feedback from Sporobolus virginicus soil demonstrates that this plant species may contribute to biotic resistance against the introduced A. arenaria, through negative feedback from the soil community. Not only the local plant species diversity, but also the type of plant species present seemed to be important in determining the potential for biotic resistance. As a result, biotic resistance against invasive plant species may depend not only on plant competition, but also on the presence of plant species that are hosts of potential soil pathogens that may negatively affect the invaders. In conclusion, exotic plant species such as A. arenaria in South Africa that do not become highly invasive, may experience the ERH and BRH simultaneously, with the balance between enemy escape versus biotic resistance determining the invasiveness of a species in a new habitat.Plant nomenclature follows Arnold and De Wet (1993)     

Experimental evidence of host race formation in Mitoura butterflies (Lepidoptera: Lycaenidae)

A population of herbivorous insects that shifts to a novel host can experience selection pressures that result in adaptation to the new resource. Host race formation, considered an early stage of the speciation process, may result. The current study investigates host shifts and variation in traits potentially involved in the evolution of reproductive isolation among populations of the juniper hairstreak butterfly, Mitoura gryneus. Mitoura are closely associated with their host trees (Cupressaceae) and exhibit host plant fidelity: in addition to larval development and oviposition, host trees support male leks and mating. Female oviposition preference for the natal host, and differential fitness of larvae when reared on natal versus alternate hosts, are indications that specialization and local adaptation to the natal host plant are occurring. Populations with single host plant associations (Juniperus ashei, J. pinchotii and J. virginiana) as well as populations with multiple hosts (both J. ashei and J. pinchotii) were examined. Concordance between female preference and larval performance was found for J. ashei‐associated populations. Population‐level variation in the patterns of female preference and larval performance, both within and among host associations, may reflect differences in the timing and direction of colonization of hosts. For a single nominal species that otherwise exhibits no morphological or phenological differences, the experimental assessment of specialization and host fidelity in M. gryneus provides strong support for the hypothesis of ongoing host race formation in these butterflies.     

Contemporary evolution of host plant range expansion in an introduced herbivorous beetle Ophraella communa

Host range expansion of herbivorous insects is a key event in ecological speciation and insect pest management. However, the mechanistic processes are relatively unknown because it is difficult to observe the ongoing host range expansion in natural population. In this study, we focused on the ongoing host range expansion in introduced populations of the ragweed leaf beetle, Ophraella communa, to estimate the evolutionary process of host plant range expansion of a herbivorous insect. In the native range of North America, O. communa does not utilize Ambrosia trifida, as a host plant, but this plant is extensively utilized in the beetle's introduced range. Larval performance and adult preference experiments demonstrated that native O. communa beetles show better survival on host plant individuals from introduced plant populations than those from native plant populations and they also oviposit on the introduced plant, but not on the native plant. Introduced O. communa beetles showed significantly higher performance on and preference for both introduced and native A. trifida plants, when compared with native O. communa. These results indicate the contemporary evolution of host plant range expansion of introduced O. communa and suggest that the evolutionary change of both the host plant and the herbivorous insect involved in the host range expansion.     

Endophytic fungus fine‐tunes the persistence strategy of its alpine host grass in response to soil resource levels

An understanding of hereditary endophytic fungi, and the effects on grass persistence strategies (i.e. relative investment in sexual reproduction and vegetative growth) under natural conditions may help to predict how some alpine ecosystems will respond to environmental change. Grass persistence and endophyte maintenance in host populations are closely related, but could become independent due to endophyte loss mechanisms. We used native grass and endophyte populations to test the hypothesis that fungal endophytes manipulate grass persistence strategies to secure endophyte maintenance in plant populations. Two conditions were required to verify this hypothesis: 1) the fungus caused alterations in host plant strategies; and 2) plant phenotypic changes induced by the fungal endophyte increased endophyte transmission. We compared symbiotic (S) and non‐symbiotic (NS) persistence strategies of Festuca eskia (Poaceae), an alpine grass infected by the asexual form of the fungal endophyte Epichloë festucae. We characterised endophyte transmission efficiency, and described vegetative growth and sexual reproduction in a field population that naturally supports approximately 50% S plants. We built a demographic model to estimate plant vegetative growth rates. A correlation between plant persistence strategy, and fungal maintenance was evaluated by increasing soil resource levels. Under natural conditions, S and NS plants exploited different persistence strategies in the same population; S plants exhibited greater vegetative growth than their NS counterparts, while maintaining the same reproductive output. In response to higher soil resource levels, S plants shifted in persistence strategies and phenology, whereas NS plants maintained the same strategies. Therefore, results suggested the fungal endophyte fine‐tuned host persistence strategies according to soil resource level. Finally, we found no direct relationship between the changes induced by fungal endophyte and endophyte transmission. Consequently, fungal endophytes affected host persistence strategies, but did not directly increase endophyte transmission.     

BEHAVIORAL EVIDENCE FOR FRUIT ODOR DISCRIMINATION AND SYMPATRIC HOST RACES OF RHAGOLETIS POMONELLA FLIES IN THE WESTERN UNITED STATES

The recent shift of Rhagoletis pomonella (Diptera: Tephritidae) from its native host downy hawthorn, Crataegus mollis, to introduced domesticated apple, Malus domestica, in the eastern United States is a model for sympatric host race formation. However, the fly is also present in the western United States, where it may have been introduced via infested apples within the last 60 years. In addition to apple, R. pomonella also infests two hawthorns in the West, one the native black hawthorn, C. douglasii, and the other the introduced English ornamental hawthorn, C. monogyna. Here, we test for behavioral evidence of host races in the western United States. through flight tunnel assays of western R. pomonella flies to host fruit volatile blends. We report that western apple, black hawthorn, and ornamental hawthorn flies showed significantly increased levels of upwind‐directed flight to their respective natal compared to nonnatal fruit volatile blends, consistent with host race status. We discuss the implications of the behavioral results for the origin(s) of western R. pomonella, including the possibility that western apple flies were not introduced, but may represent a recent shift from local hawthorn fly populations.     

GENETIC DIVERSITY AND POPULATION STRUCTURE IN TRADESCANTIA HIRSUTICAULIS (COMMELINACEAE)

Tradescantia hirsuticaulis, the hairy-stemmed spiderwort, is an insect-pollinated perennial plant species found primarily on rock outcrops in Georgia, South Carolina, and Alabama. Although populations of T. hirsuticaulis are rare and scattered, local populations are frequently large. Levels of genetic variation were assessed for 13 populations representing the species' range in these three states. Despite the disjunct distribution of this habitat specialist and apparent lack of specialized seed and pollen dispersal mechanisms, exceptionally high levels of genetic variation are maintained within the species, with a moderate level of variation (18%) found among populations. Twenty-nine of the 33 loci resolved (88%) were polymorphic within the species; the mean number of loci polymorphic within populations was 54%. The mean number of alleles per polymorphic locus was 3.24 across all populations and averaged 2.37 within populations. Genetic diversity was 0.206 for the species, whereas mean population genetic diversity was 0.157, both much higher than the average for other short-lived herbaceous perennials. Estimated levels of gene flow were moderate (Nm = 0.95), and a significant association between geographic distance and genetic distance between populations was found (r = 0.68; P < 0.0001). Habitat destruction is the major threat to this genetically diverse species. Since gene flow among its highly dispersed populations is limited, diminution or extinction of local populations could jeopardize the long-term evolutionary potential of this species.